Basement Bathroom Below the Sewer Outfall
A 1950s two-story house has a 4-inch cast-iron stack serving the first-floor water closet and the second-floor bathroom group. The owner wants a basement bathroom - water closet, lavatory and shower - directly below the first-floor bathroom. The basement slab is 7 ft 6 in. below the first floor; the building drain leaves the building with its invert 6 ft 8 in. below the first floor, and the developed run from the proposed fixture location to that drain is about 25 ft. The sewer invert at the property line, 60 ft away, is 7 ft 4 in. below the first floor. The contractor proposes 2-inch pipe from the new group into the horizontal building drain, wet-venting the new water closet through the existing 1½-inch lavatory drain, and terminating the lavatory vent with an air admittance valve buried in a finished wall cavity so no new roof penetration is needed.
1. Can the new basement bathroom drain to the existing building drain by gravity? Show the elevation arithmetic and state what the installation must include if it cannot.
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Do the arithmetic before discussing anything else.
- Basement slab: 7 ft 6 in. below first floor.
- Water closet outlet on that slab: roughly 1 ft above the slab, i.e. about 6 ft 6 in. below the first floor.
- Building drain invert at the foundation: 6 ft 8 in. below the first floor - only 2 inches below the fixture outlet.
- A 2-inch drain needs 1/4 in./ft. Over the 25-ft developed run that is about 6¼ inches of fall, and there is only 2.
You cannot make it up downstream either: the sewer invert at the property line is 7 ft 4 in. below the first floor, so there is no additional depth to buy. Gravity drainage is out.
The correct answer is a sewage ejector/lift station: a gas-tight receiving basin set below the slab, a duplex or single pump sized for the three-fixture group's discharge, a check valve and a full-port gate or ball valve on the discharge, and a high-water alarm. The basin must be vented independently through the roof - an ejector vent cannot be an air admittance valve, and it must not be combined with the fixture vents below the flood level rim. The discharge ties into the building drain only after the check valve, and the tie-in point must be arranged so the pumped flow cannot back up into the new fixtures. If the building drain is subject to surcharge, a backwater valve is also required.
2. Evaluate the proposed wet vent and the air admittance valve on the lavatory. What is compliant, what is not, and what would you require instead?
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Wet vent - not compliant. A water closet may be wet-vented by a lavatory only where the wet-vented section is sized for the load it carries; for a water closet that means a 2-inch minimum wet vent, with the lavatory draining into it from above and no other fixtures downstream. Here the lavatory drain is 1½ inch, so it cannot function as the wet vent for a water closet. The 2-inch line the contractor proposed to run to the building drain is a drain, not a vent.
Adverse slope / connection detail - check it. A vent must rise; if the new water closet is back-vented, the vent connection must be taken off the top of the drain and rise without a horizontal offset below the flood level rim of the fixture.
Air admittance valve - not acceptable here. Three independent problems: (1) AAVs must be listed, installed in an accessible location and in accordance with their listing; sealing one inside a finished wall cavity fails that. (2) An AAV is not a substitute for a vent through the roof where the drainage system depends on open-air relief, and it cannot vent a water closet or a pump basin. (3) If the local amendment limits AAVs to individual fixture vents with at least one open vent through the roof, the installation must be checked against that amendment.
What to require: a 2-inch vent for the new water closet taken to the existing vent stack (verify the stack vent is large enough for the added load, or run a new vent through the roof), a 1½-inch vent for the lavatory, a 1½-inch vent for the shower, and re-venting above the highest flood level rim in the group.
3. The homeowner wants to skip the permit because 'it's just a bathroom in the basement.' How do you handle it, and what must the permit drawings show?
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The work adds three fixtures, a new drain connection, a pump basin and a new vent penetration - it is not exempt minor work, and the ejector basin is a special item the inspector will want to see. Doing it unpermitted also leaves the homeowner exposed when the finished basement is later sold.
Minimum submittal:
- Floor plan with fixture locations and fixture-unit counts for the new group; confirm the 4-inch building drain and the stack still carry the added load.
- Isometric showing the ejector basin, pump discharge, check valve, valve, and the point of connection to the building drain.
- Venting plan: sizes, routing, tie-in to the existing vent stack or a new roof penetration, and confirmation the stack vent is adequately sized.
- Electrical data for the pump and alarm and confirmation of a dedicated circuit and GFCI protection as required.
- Structural note for the slab cut and the basin backfill.
Flag to the owner in writing: the wet vent as proposed and the buried AAV will both be rejected, and the ejector is unavoidable because of the elevations.